The invention relates to nuclear reactor dip seal maintenance systems and more particularly to liquid sodium dip seal maintenance systems of nuclear reactor components having rotatable members.
In nuclear reactor designs, well known in the art, a reactor vessel with fuel assemblies disposed therein and having an inlet and an outlet for circulation of a coolant in heat transfer relationship with the fuel assemblies is sealed by a closure head located on top of the reactor vessel. In certain designs, the closure head comprises one or more rotatable plugs which may be of varying sizes disposed eccentrically within each other. Moreover, the rotatable plugs may have refueling equipment disposed on them. In these designs, a selected rotation of the rotatable plugs serves to position the refueling machines in appropriate relationship to the fuel assemblies in the reactor vessel to facilitate the refueling process.
Since the rotatable plugs must be able to rotate relative to each other, the plugs are mounted so as to define an annulus between them that may extend from the bottom to the top of the closure head. However, while the annulus provides for the rotation of the plugs, it also establishes a path for the release of radioactive particles that may be present in a cover gas that fills the void between the bottom of the closure head and the top of the reactor coolant pool. Accordingly, seals are provided at various locations across the annuli to prevent this release of radioactive particles. These seals may be of various types located at one or more locations along the annulus. As is well known in the art, a typical annulus seal may be a liquid dip seal. Furthermore, in a liquid metal fast breeder reactor, the liquid used in the dip seal may be sodium. In certain applications, the liquid sodium dip seal may be placed at such a location or with heating elements surrounding it so that the sodium remains in a liquid state during reactor operation. In other applications, the sodium in the dip seal is allowed to solidify during reactor operation, thereby increasing the effectiveness of the seal. However, in any application the sodium must be a liquid during reactor refueling to enable the plugs to rotate.
During reactor operation, the cover gas which may be an inert gas such as argon becomes contaminated with not only radioactive particles but also oxides and hydrides of sodium. Because the sodium in the dip seal is in contact with the cover gas and gas above the seal, the sodium in the dip seal also becomes contaminated with these compounds and other impurities. These impurities may cause a crust to form on the sodium dip seal free liquid surface and may be deposited on the metal surfaces bonding the liquid sodium. In addition, the amount of liquid sodium within the dip seal is reduced due to the formation of these impurities. These impurities may continue to develop to the extent of resisting rotation of the plugs or even to the extent of preventing this rotation. Therefore, it is necessary to periodically remove the impurities from the dip seals. Several methods and mechanisms are known for removing these kinds of impurities from components but they have not been effective when used in reactor operations.
One method for removing impurities from sodium which is well known in the art involves the application of steam to the deposits. The steam causes a chemical reaction which causes the impurities to recombine and be forced out of the dip seal. There is, however, a major problem with this method. This problem is that liquid sodium reacts violently in contact with steam or oxygen. To reduce this reaction, the steam is mixed with an inert gas such as argon in such a percentage as to limit this violent reaction. Nevertheless, the use of steam creates the same kind of problems that it seeks to eliminate because the steam itself adds impurities that may combine with sodium elsewhere in the reactor system such as the reactor coolant area or other annuli thereby creating a similar problem at a different location. Therefore, while the steam method may temporarily clean the dip seals, the problem may recur at another location.
The most common method for removal of the crust associated with these impurities is by a contact tool such as a scraper. In this method, a scraper is placed in contact with the crust while the plugs are rotated thereby scraping the crust loose and preventing its accumulation to the extent of preventing rotation of the plugs. However, in this method, the scraping action may accumulate the crusty impurities between the scraper and plug annuli surfaces so as to clog the annuli and prevent further rotation of the plugs. Of course, this difficulty prevents this method from being a satisfactory method of removing these impurities.
In U.S. Pat. No. 3,819,478, to Thorel et al, issued June 25, 1974, there is described an apparatus that attempts to provide a pair of properly located redundant dip seals in a top shield plug annulus that prevents liquid sodium vapors or cover gas from entering the annulus below the top frozen dip seal in a rotating plug. The Thorel patent also attempts to describe a means of removing impurities from sodium before the sodium is introduced into the dip seals. While the Thorel patent may describe one method of removing impurities from sodium outside of a dip seal it does not teach a method of removing impurities that may contaminate the sodium in a dip seal that is not easily accessible.
While the prior art contains several methods for cleaning sodium deposits on reactor components and for removing impurities from sodium, it does not teach an effective method or effective non-contacting tool for removing impurities from relatively inaccessible reactor components, such as liquid sodium dip seals. In addition to the problem of impurities in the liquid of the dip seal forming a crust that hampers rotation, the contaminated cover gas also causes a problem by migrating through small gaps between the liquid sodium in the dip seals and the metal surfaces of the dip seals thereby contaminating the atmosphere beyond the dip seals.
Normal machining of components for nuclear reactors, while being extremely precise, nevertheless, leave minute ridges on the machined surfaces. The manufacture of rotatable closure head plugs along with the metal surfaces that comprise the dip seals, likewise, leaves ridges on the surfaces of the metal that comprise the dip seals. With liquid sodium in the dip seals, the metal surfaces below the liquid level of the dip seal appear to come in contact with the liquid sodium. This contact between the metal surfaces and the liquid sodium is normally thought to form a barrier against the escape of contaminated cover gas past the dip seal. However, tests have shown that while the liquid sodium in the dip seals does contact the crest of the ridges in the machined metal, surface tension effects well known to those skilled in the art prevent the liquid sodium from completely filling all of the grooves formed by the machined ridges in the metal surfaces. The result of this less than total filling of grooves is that minute amounts of radioactive particles contained in the contaminated cover gas trace a somewhat tortuous leak path through these grooves and escape past the liquid dip seal thereby contaminating the atmosphere beyond the dip seal. In order to eliminate these leak paths and thus significantly reduce radioactive leakage past the dip seals, it is necessary that these grooves become totally filled with liquid sodium. A surface that has these types of grooves filled with a liquid is referred to in the art as a wetted surface.
There are many methods known in the art for reducing surface tension effects so that a metal surface may become wetted with a liquid. One such method is to coat the machined surface with another metal such as gold so that when the coated surface comes into contact with the liquid, the liquid causes the metal coating to dissolve in the liquid while wetting the metal surface. While this method and others related to it may be effective in an application wherein the metal surfaces are small and easily accessible, it is not feasible for use in applications where the metal surfaces are large and inaccessible such as in liquid dip seals of nuclear reactor components. Moreover, in nuclear reactor components wherein the contaminated cover gas causes impurities to form in the dip seal, the impurities can accumulate in the grooves so as to cause a once wetted surface to become non-wetted. Therefore, it is necessary to rewet the metal surfaces after the reactor has been operating for a period of time. It is this rewetting that necessitates an apparatus capable of wetting the metal surfaces of the dip seals while the metal surfaces remain in a location where accessibility is limited.
There are, therefore, at least two problems associated with the contamination of the liquid sodium in the dip seals, one being the formation of a crust in the dip seal and the other being the leakage of radioactive particles through minute grooves in the metal surfaces of the dip seal. Although there are several methods known in the art for rectifying these problems, those methods are not effective where accessibility to the problem area is limited such as in components of nuclear reactors.